1. Field of the Invention
The present invention relates to a zoom lens and imaging apparatus. More specifically, with respect to a zoom lens that is capable of image shifting, the present invention relates to a technology that makes it possible to reduce the lens diameter, and that suppresses fluctuations in various aberrations that occur during image shifting.
2. Description of Related Art
With a zoom lens that has a large zoom ratio, because the angle of field becomes narrow in the maximum telephoto state, there is an issue in that even the smallest hand movement or camera shake causes a significant blurring of the image.
As methods of image stabilization that correct for the blurring of images caused by such hand movements or the like, optical image stabilization systems are known.
With an optical image stabilization system, by combining a detection system that detects movements of the camera that accompany hand movements that result from shutter release, a control system that instructs a displacement to a drive system based on a signal outputted from the detection system, the drive system that drives an optical system based on an output from the control system, and the optical system that is capable of image shifting when a displacement is instructed by the drive system, it is possible to correct for the blurring of the image caused by movements of the camera by canceling it out with the blurring of the image that accompanies the displacement of the optical system.
For the optical system used in such optical image stabilization systems, such methods as the lens shift method where a portion of the lens system is moved in a direction that is perpendicular to the optical axis, or the variable angle prism method where the angle of the prism located right before the lens system is varied.
In the lens shift method, in order to suppress changes in the optical performance that occur when a predetermined lens is shifted, the number of lenses increases. Because the lenses need to be suspended midair (in a neutral position), there is an issue in that reducing power consumption is difficult.
The variable angle prism method does not have this issue that is seen in the lens shift method.
The variable angle prism method can be divided into two types. One is where a variable angle prism is placed on the side of the optical system that is closest to the object, and the other is where a variable angle prism is placed in the optical system itself.
An example where a variable angle prism is placed on the side of the optical system that is closest to the object can be seen in the optical system disclosed in Japanese Patent Application Publication No. S62-153816 (Patent Document 2).
A case where a variable angle prism is placed on the side of the optical system, which is closest to the object, is characteristic in that the change in the angle of field in relation to the amount, by which the angle of the variable angle prism is varied, is not dependent on the focal length of the zoom lens. When image blurring caused by camera movements that accompany hand movements and the like are to be corrected, it is advantageous in that the angle of the variable angle prism can be controlled independent of the focal length of the zoom lens. On the other hand, however, since image blurring is amplified in the maximum telephoto state, as the zoom ratio becomes greater, the stopping in the maximum telephoto state needs to be controlled with higher accuracy, and there is an issue in that higher precision is demanded for the drive system.
In addition, since the variable angle prism is placed on the object side of the first lens group, whose lens diameter is large, the diameter of the beam of light that is incident upon the variable angle prism is large, and as a result, there is an issue in that the variable angle prism itself becomes very large, and further in that protective measures, such as providing a protective glass and the like, need to be taken in order to prevent the user's hand from coming into direct contact with the variable angle prism. For these reasons, there is a limit as to the extent to which miniaturization can be carried out.
In contrast, examples where a variable angle prism is located within the optical system are disclosed in Japanese Patent Application Publication No. H02-168223 (Patent Document 1), Patent Document 2, Japanese Patent Application Publication No. H10-246855 (Patent Document 3), Japanese Patent Application Publication No. H11-44845 (Patent Document 4) and the like.
In the optical system disclosed in Patent Document 1, a variable angle prism is provided where the beam of light is parallel. In the optical system disclosed in Patent Document 2, a variable angle prism is provided before the master lens group. In the optical systems disclosed in Patent Document 3 and Patent Document 4, a variable angle prism is provided near the aperture stop.
With the optical systems disclosed in Patent Documents 1 through 4, since the axial beam of light is incident upon the variable angle prism in a state where it is close to being parallel, they are characteristic in that fluctuations in axial aberration that occur when the prism angle is varied can be reduced.